CN111395031A

CN111395031A - Method for synchronously preparing cellulose nanowhiskers and cellulose nanofibrils

Info

Publication number: CN111395031A
Application number: CN202010210110.6A
Authority: CN
Inventors: 徐峻; 王佳佳; 王斌; 高文花; 李军; 曾劲松; 陈克复
Original assignee: South China University of Technology SCUT
Current assignee: South China University of Technology SCUT
Priority date: 2020-03-23
Filing date: 2020-03-23
Publication date: 2020-07-10

Abstract

The invention discloses a method for synchronously preparing cellulose nanowhiskers and cellulose nanofibrils. The preparation method comprises the following steps: and carrying out hydrolysis reaction on the paper pulp by using a low-concentration sulfuric acid solution at high temperature to obtain a cellulose nanowhisker suspension, and homogenizing the separated cellulose solid residue by using a high-pressure homogenizer to obtain the cellulose nanofibril. The preparation method can fully utilize fiber raw materials, the conversion rate is close to 100%, the prepared nano-cellulose has higher length-diameter ratio, the initial thermal degradation temperature of the nano-cellulose is higher than that of the nano-cellulose prepared by the traditional concentrated sulfuric acid method by about 100 ℃, and the nano-cellulose has great advantages in the application aspect of nano-composite materials.

Description

Method for synchronously preparing cellulose nanowhiskers and cellulose nanofibrils

Technical Field

The invention belongs to the field of cellulose materials, and particularly relates to a method for synchronously preparing cellulose nanowhiskers and cellulose nanofibrils.

Background

Cellulose is the most potential renewable biomass material on the earth and is widely applied to the fields of papermaking, daily chemicals, textiles and the like. Nanocellulose has attracted much attention for its excellent properties (high crystallinity, low density, large specific surface area, high strength, etc.). Many high-value byproducts can be derived from the bio-based material of nano-cellulose, such as hydrogel, aerogel, nano-composite, optoelectronic materials, etc.

Currently, there are many methods for preparing nanocellulose, such as concentrated sulfuric acid method, high pressure homogenization method, TEMPO oxidation method, etc., but all of the above methods have certain disadvantages, for example, patent CN105625077A adopts concentrated sulfuric acid hydrolysis method to prepare nanocellulose, which uses a large amount of sulfuric acid and needs subsequent dialysis treatment, not only the period is long, but also a large amount of acid-containing wastewater is generated. The improvement on this process still requires subsequent treatment of large amounts of waste acid, such as (meth) sulfonic acid instead of part of sulfuric acid in patent CN109208366A, and also subsequent removal of acid liquor. The high-pressure homogenization method has the defect of high energy consumption, and the TEMPO oxidation method has the defects of complicated method and the like. Meanwhile, the method has low fiber conversion rate and invisibly causes raw material waste. In order to solve the problem, the scholars perform homogenization treatment on the residual residue left by concentrated sulfuric acid hydrolysis to obtain cellulose nanofibrils, and meanwhile, in order to solve the problem of subsequent treatment of a large amount of waste acid, the scholars use dilute sulfuric acid to pretreat slurry and perform high-pressure homogenization to obtain high-yield cellulose nanofibrils, but cellulose nanowhiskers are not obtained, and reducing sugar in hydrolysate is not comprehensively utilized. Patent CN106883301B adopts wet grinding to prepare cellulose nanocrystals and cellulose nanofibrils at the same time, and this method has high conversion rate, but has the problems of high energy consumption and poor product dispersibility. Therefore, the method needs to solve the problems of complex process, subsequent treatment of waste acid and the like in the traditional method for preparing the nano-cellulose, and simultaneously obtains the cellulose nano-whiskers and the cellulose nano-fibrils, so that the method for realizing the full utilization of raw materials is very necessary.

Disclosure of Invention

The invention aims to overcome the defects of concentrated acid hydrolysis of plant fibers and provide a method for synchronously preparing cellulose nanowhiskers and cellulose nanofibrils. The method uses an ultralow-concentration sulfuric acid solution to hydrolyze a fiber raw material at a higher temperature to obtain cellulose nanowhiskers and cellulose nanofibrils; meanwhile, residual acid liquor is removed by means of filtration, so that the reaction period is shortened, and the trouble of a large amount of waste acid is avoided. The method has the advantages of high utilization rate of raw materials, extremely small using amount of sulfuric acid and less wastewater pollution, and provides a new green and environment-friendly way for realizing synchronous preparation of the cellulose nanowhiskers and the cellulose nanofibrils.

The purpose of the invention is realized by the following technical scheme.

A method of simultaneously preparing cellulose nanowhiskers and cellulose nanofibrils, comprising the steps of:

(1) hydrolysis: adding the slurry into dilute acid solution, and carrying out hydrolysis reaction at 110-180 ℃; the concentration of the dilute acid solution is 0.1-0.4 wt%;

(2) filtering and washing: removing reducing sugar in the hydrolysate by using a filter membrane from the reactant obtained in the step (1), collecting filtrate, and filtering and washing the residual filter residue to be neutral by using water again;

(3) centrifuging: centrifuging the filter residue obtained in the step (2), collecting upper-layer suspension, continuously and repeatedly adding water into the residue for washing, centrifuging, collecting the upper-layer suspension until the upper-layer suspension becomes clear, and collecting solid residue; the obtained upper layer suspension is cellulose nano whisker suspension;

(4) homogenizing: and (4) putting the solid residue obtained in the step (3) into water, and homogenizing to obtain the cellulose nanofibrils.

Preferably, the slurry in the step (1) is obtained by pulp dissociation; the paper pulp is common paper pulp board, dissolving pulp board or wet paper pulp.

Further preferably, the pulp is a common bleached softwood pulp sheet.

Preferably, the dilute acid in step (1) is dilute sulfuric acid.

Preferably, the concentration of the diluted acid solution in the step (1) is 0.25-0.35 wt%.

Preferably, the adding amount of the slurry in the step (1) is 3-7% of the mass of the dilute acid solution.

Further preferably, the adding amount of the slurry is 3-4% of the mass of the dilute acid solution.

Preferably, the temperature of the hydrolysis reaction in step (1) is 150-160 ℃.

Preferably, the hydrolysis reaction time in the step (1) is 1-5 h.

Further preferably, the time of the hydrolysis reaction is 2 to 3 hours.

Preferably, the filter membrane in the step (2) is a micro-nano filter membrane.

Preferably, in step (4), the solid residue obtained in step (3) is put into water to prepare a 0.4wt% solution.

Compared with the prior art, the invention has the following advantages and effects:

1. the invention can simultaneously obtain the cellulose nano crystal whisker and the cellulose nano fibril by combining the hydrolysis method of the ultra-low concentration sulfuric acid solution with the assistance of homogenization, and the total yield of the two types of nano cellulose is up to more than 80 percent.

2. The method has the advantages of low acid consumption, and the acid consumption is one percent of that of the conventional method. The hydrolysate is filtered and washed by a filter membrane to obtain neutral cellulose solid residue, so that the time consumed by dialysis is greatly shortened.

3. The cellulose nano crystal whisker prepared by the method has high thermal degradation starting temperature which is 100 ℃ higher than that of nano cellulose prepared by a conventional sulfuric acid method, and provides great advantages for the application of the cellulose nano crystal whisker in the field of nano composite materials.

4. The invention considers the contents of reducing sugar, glucose and fructose in the hydrolysate to be about 15-20 percent, and realizes the full utilization of the raw materials.

Drawings

FIGS. 1a and 1b are TGA and DTG thermogravimetric analysis graphs of cellulose nanowhiskers prepared under different acid hydrolysis conditions.

Figure 2 is an SEM image of cellulose nanowhiskers prepared under the conditions of example 1.

Figure 3 is a graph of a statistical analysis of the size of cellulose nanowhiskers under the conditions of example 1.

FIG. 4 is an SEM photograph of cellulose nanofibrils obtained by homogenizing the residue under the conditions of example 2.

Figure 5 is an SEM image of cellulose nanowhiskers prepared under the conditions of comparative example 1.

Detailed Description

The present invention will be described in further detail below with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.

Example 1

The sulfuric acid concentration was 0.3 wt%, i.e. sulfuric acid: water accounts for 0.3 percent of the mass ratio;

fiber concentration was 3 wt%, i.e. fiber: 3 percent of water and fiber;

1.5g of plant fiber slurry and 48.5m L sulfuric acid solution are uniformly mixed, then the mixture is reacted for 2h at 160 ℃ through a hydrothermal reaction kettle, when the temperature is stable, the timing is started, after the reaction is finished, hydrolysate is filtered and is reserved for HP L C detection, the residual solid fiber is centrifuged for 15min at 10000rpm, a cellulose Nano whisker suspension is obtained, the morphology of the cellulose Nano whisker suspension is characterized through a scanning electron microscope, as shown in figure 2, the cellulose Nano whisker is in a rod-shaped structure, the length of the cellulose Nano whisker is mainly distributed between 200 and 400nm, the size statistics of the suspension obtained under the condition of the embodiment 1 is carried out through a software Nano Meiaser 1.2, the cellulose Nano whisker is 500nm in length and 15-35nm in diameter (shown in figure 3), and the residual cellulose solid residue is homogenized for 6 times under high pressure, so that the yield of the Nano cellulose Nano whisker is about 69.11%.

Example 2

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

uniformly mixing 2g of plant fiber slurry with 48m of L sulfuric acid solution, reacting for 2h at 160 ℃ through a hydrothermal reaction kettle, starting timing when the temperature is stable, filtering hydrolysate after the reaction is finished, detecting HP L C, centrifuging for 15min at 10000rpm to obtain cellulose nanowhisker suspension, and homogenizing the residual cellulose solid residue under high pressure for 6 times to obtain the nanocellulose fibrils, wherein the length is about 400-1200 nm and the yield is about 68.67% as shown in figure 4.

Example 3

The sulfuric acid concentration was 0.35wt%, i.e. sulfuric acid: water accounts for 0.35 percent of the mass ratio;

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

2g of plant fiber slurry and 48m of L sulfuric acid solution are mixed uniformly and then react for 3h at 150 ℃ through a hydrothermal reaction kettle, timing is started when the temperature is stable, after the reaction is finished, hydrolysate is filtered and reserved for HP L C detection, then cellulose nanowhisker suspension is obtained after centrifugation is carried out for 15min at 10000rpm, and the cellulose nanowhisker suspension is obtained after the rest cellulose solid residue is homogenized for 6 times under high pressure, and the yield is about 67.42%.

Example 4

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

2g of plant fiber slurry and 48m of L sulfuric acid solution are mixed uniformly and then react for 3h at 160 ℃ through a hydrothermal reaction kettle, timing is started when the temperature is stable, after the reaction is finished, hydrolysate is filtered and reserved for HP L C detection, then cellulose nanowhisker suspension is obtained after centrifugation is carried out for 15min at 10000rpm, and the rest cellulose solid residue is homogenized for 6 times under high pressure to obtain the nanocellulose fibrils, wherein the yield is about 64.42%.

Example 5

The sulfuric acid concentration was 0.25 wt%, i.e. sulfuric acid: water accounts for 0.25 percent of the mass ratio;

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

the preparation method comprises the steps of uniformly mixing 2g of plant fiber slurry with 48m of L sulfuric acid solution, reacting for 3 hours at 160 ℃ through a hydrothermal reaction kettle, starting timing when the temperature is stable, filtering hydrolysate after the reaction is finished, detecting HP L C, centrifuging for 15 minutes at 10000rpm to obtain cellulose nanowhisker suspension, and homogenizing residual cellulose solid residues for 6 times under high pressure to obtain the nanocellulose fibrils, wherein the yield is about 70.39%.

Example 6

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

uniformly mixing 2g of plant fiber slurry with 48m of L sulfuric acid solution, reacting for 3h at 180 ℃ through a hydrothermal reaction kettle, starting timing when the temperature is stable, filtering hydrolysate after the reaction is finished, reserving HP L C for detection, centrifuging for 15min at 10000rpm, and obtaining no cellulose nanowhisker suspension, and homogenizing the residual cellulose solid residue for 6 times under high pressure to obtain the nanocellulose fibers, wherein the yield is about 62.48%.

Example 7

The sulfuric acid concentration was 0.1 wt%, i.e. sulfuric acid: water accounts for 0.1 percent of the mass ratio;

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

the preparation method comprises the steps of uniformly mixing 2g of plant fiber slurry with 48m of L sulfuric acid solution, reacting for 3 hours at 160 ℃ through a hydrothermal reaction kettle, starting timing when the temperature is stable, filtering hydrolysate after the reaction is finished, reserving HP L C for detection, centrifuging for 15 minutes at 10000rpm, obtaining no cellulose nanowhisker suspension, and homogenizing residual cellulose solid residues for 6 times under high pressure to obtain the nanocellulose fibers, wherein the yield is about 76.84%.

Example 8

The sulfuric acid concentration was 0.4wt%, i.e. sulfuric acid: water accounts for 0.4 percent of the mass ratio;

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

the preparation method comprises the steps of uniformly mixing 2g of plant fiber slurry with 48m of L sulfuric acid solution, reacting for 3 hours at 160 ℃ through a hydrothermal reaction kettle, starting timing when the temperature is stable, filtering hydrolysate after the reaction is finished, reserving HP L C for detection, centrifuging for 15 minutes at 10000rpm, obtaining no cellulose nanowhisker suspension, and homogenizing residual cellulose solid residues for 6 times under high pressure to obtain the nanocellulose fibers, wherein the yield is about 67.03%.

Example 9

fiber concentration was 4wt%, i.e. fiber: 4 percent of water and fiber;

uniformly mixing 2g of plant fiber slurry with 48m of L sulfuric acid solution, reacting for 3h at 110 ℃ through a hydrothermal reaction kettle, starting timing when the temperature is stable, filtering hydrolysate after the reaction is finished, and reserving for HP L C detection, and centrifuging for 15min at 10000rpm to obtain cellulose nanowhisker suspension.

Comparative example 1

The sulfuric acid concentration was 64 wt%, i.e. sulfuric acid: water accounts for 64 percent in mass ratio;

10g of commercial CMC was reacted with 100ml of sulfuric acid in admixture, at a temperature of 45 ℃ for 45min by magnetic stirring, and the reaction was stopped by adding 10 times the volume of water. After the suspension is dialyzed to be neutral, the nano-cellulose is obtained by centrifugation, as shown in figure 5, the main length range is between 100 nm and 200nm, and the diameter is between 3 nm and 10 nm.

The cellulose nanowhiskers obtained in examples 1 to 8 were whisker-like cellulose having a diameter of 15 to 50nm and a length of 150 to 700 nm; the cellulose nano-fibril has the size of 15-50 nm in diameter and 400-970 nm in length; the byproducts are hydrolyzed glucose and converted fructose. After hydrolysis, centrifuging for 15min under the condition of 10000rpm, and separating to obtain a light blue suspension liquid as the cellulose nano crystal whisker on the upper layer; and (4) the rest precipitates are micro-nano cellulose residues, and the cellulose nanofibrils can be obtained after the residues are further homogenized.

TABLE 1 summary of hydrolysate yields for each example

Thermogravimetric analysis graphs of the cellulose nanowhiskers obtained under the conditions of examples 1 to 5 and comparative example 1 are shown in fig. 1a and 1b, and the cellulose nanowhiskers obtained under the conditions of examples 1 to 5 have an initial thermal degradation temperature 100-120 ℃ higher than that of the cellulose nanowhiskers obtained under the condition of comparative example 1.

As shown in FIG. 3, under the preferred conditions (example 1), the particle size distribution range of the obtained cellulose nanowhiskers is 150-500nm, which is mainly concentrated between 200-400 nm.

The above examples are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above examples, and any other simple changes to the process (including changing the hydrolysis apparatus or changing the experimental raw materials, etc.) without departing from the principle of the experiment are within the scope of the present invention.

Claims

1. A method for simultaneously preparing cellulose nanowhiskers and cellulose nanofibrils, comprising the steps of:

2. The method according to claim 1, wherein the slurry of step (1) is obtained by pulp dissociation; the paper pulp is common paper pulp board, dissolving pulp board or wet paper pulp; the dilute acid is dilute sulfuric acid.

3. The process of claim 1, wherein the dilute acid solution of step (1) has a concentration of 0.25 to 0.35 wt.%.

4. The method of claim 1, wherein the slurry is added in step (1) in an amount of 3-7% by mass of the dilute acid solution.

5. The method according to claim 4, wherein the slurry is added in an amount of 3-4% by mass of the dilute acid solution.

6. The method as claimed in claim 1, wherein the temperature of the hydrolysis reaction in step (1) is 150-160 ℃.

7. The method according to claim 1, wherein the hydrolysis reaction time in step (1) is 1-5 h.

8. The process according to claim 7, characterized in that the hydrolysis reaction time is between 2 and 3 h.

9. The method according to claim 1, wherein the filter membrane of step (2) is a micro-nano filter membrane.

10. The method according to claim 1, wherein in the step (4), the solid residue obtained in the step (3) is put into water to prepare a solution with a concentration of 0.4 wt%.